(1) Field of the Invention
The present invention relates to an olefin resin composition for injection molding. More particularly, the present invention relates to an olefin resin composition for injection molding, which comprises an ultra-high-molecular-weight polyethylene, can be injection-molded and has excellent sliding characteristics, excellent abrasion resistance, improved heat resistance and improved rigidity in combination.
(2) Description of the Related Art
Ultra-high-molecular-weight polyethylene is excellent in impact strength, abrasion resistance, chemical resistance and tensile strength over general-purpose polyethylene and the application field of ultra-high-molecular-weight polyethylene as an engineering plastic material is being expanded. However, ultra-high-molecular-weight polyethylene has a much higher melt viscosity and a lower flowability than general-purpose polyethylene, and therefore, molding of ultra-high-molecular-weight polyethylene by ordinary extrusion molding or injection molding is very difficult. Accordingly, ultra-high-molecular-weight polyethylene is practically molded by compression molding, and on rare occasions, the polymer is molded into rods or the like at a low speed by extrusion molding.
If ultra-high-molecular-weight polyethylene inferior in the melt flowability is molded by injection molding, a shear fracture flow is generated while the resin is filled in a cavity of a mold and mica-like delamination is caused in the molded body, and therefore, a molded article exerting excellent characteristics of ultra-high-molecular-weight polyethylene cannot be obtained. Namely, the molded article is inferior to a molded article of general-purpose polyethylene.
As the injection molding method in which delamination is not caused, we previously proposed a method in which before injection molding of the resin or after completion of injection molding, the volume of a mold cavity is slightly increased and the mold is compressed to a predetermined volume (see Japanese Patent Publication No. 57-30067 and Japanese Patent Publication No. 60-58010). If this method is adopted, an injection-molded article having inherent excellent characteristics of ultra-high-molecular-weight polyethylene, such as high impact strength and high abrasion resistance, can be obtained without occurence of delamination. However, an injection molding machine provided with a mechanism for changing the volume of a mold cavity should be used for carrying out injection molding according to this method, and an injection molding machine for general-purpose polyethylene cannot be directly used.
As the method for improving the melt flowability of an ultra-high-molecular-weight polyolefin, there have been proposed various methods in which an ultra-high-molecular-weight polyolefin, is mixed with a low-molecular-weight or high-molecular-weight polyolefin.